Pigmentation disorders and tanning play a significant role in skin appearance and the sense of well being. Many people feel that they look better with tanned skin, even though achieving such an appearance may be contributing in the long term to the formation of pigment disorders. In some cultures, such as in Asia, pigmentation concerns outweigh worries about developing wrinkles. Like acne, disorders of pigmentation cause significant stress and embarrassment, so the treatment options should be understood by every cosmetic physician. In this chapter, the mechanisms known to be involved in pigment formation will be explained and the pigmentary conditions most likely to be seen by a cosmetic dermatologist will be discussed. There is a wide array of rare dyschromias that are more pathologic in nature and that are beyond the scope of this chapter. Cosmetic dermatologists are often faced with patients presenting with melasma, solar lentigos, postinflammatory hyperpigmentation, and circles under the eyes. This group of conditions will be focused on here, in addition to some treatment options. Depigmenting agents will be discussed in greater detail in Chapter 33.
Skin color results from the incorporation of the melanin-containing melanosomes, produced by the melanocytes, into the keratinocytes in the epidermis and their ensuing degradation. Although other factors contribute to skin color, such as carotenoids or hemoglobin,1 the amount, quality, and distribution of melanin present in the epidermis is the main source responsible for human skin color. The number of melanocytes in human skin is equal in all races, thereby rendering melanocyte activity and interaction with the keratinocytes as the accountable factors for skin color.2 In darker-pigmented individuals, melanocytes produce more melanin; the melanosomes are larger and more heavily melanized, and they undergo degradation at a slower rate than in lighter-skinned individuals.3 Skin of color will be further discussed in Chapter 14.
Melanin pigment is produced in the melanosome, an organelle located in the cytoplasm of the melanocytes. Melanosomes in human skin undergo four stages of development while inside the melanocyte. In stage I, premelansomes are characterized by their spherical structure and amorphous matrix. During stage II, they become more oval shaped with no apparent melanin. In stage III, following tyrosinase activity, melanin production starts and the melanization continues to stage IV, at which point the organelle contains high concentrations of melanin. The melanosomes are then transferred along microtubules to the dendritic structures of melanocytes and transferred to the keratinocytes.
The process of melanin production in the melanosomes is conducted via a pathway that begins with the hydroxylation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) using the enzyme tyrosinase, which subsequently oxidizes DOPA to dopaquinone, leading to the formation of melanin4 (Fig. 13-1). Two types of melanin are produced: eumelanin and pheomelanin. The relative amounts of these two types determine hair color ...